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Some of the most important Physics Topics include energy, motion, and force.
Effects of refraction of light with diagram
The refraction of light produces many effects which can be easily observed in our day to day life. We will now describe some of the important effects of the refraction of light. It is due to the refraction of light that:
- a stick (or pencil) held obliquely and partly immersed in water appears to be bent at the water surface.
- an object placed under water appears to be raised.
- a pool of water appears to be less deep than it actually is.
- when a thick glass slab is placed over some printed matter, the letters appear raised when viewed from the top.
- a lemon kept in water in a glass tumbler appears to be bigger than its actual size, when viewed from the sides.
- the stars appear to twinkle on a clear night.
All these effects are produced by the refraction of light (or change in direction of light) when it passes from one medium to another. Let us discuss some of these effects of refraction in detail.
1. Stick Partly Immersed in Water Appears to be Bent at the Water Surface
When a straight stick is partly immersed in water and held obliquely to the surface, it appears to be bent at the point where it enters water (see Figure). This apparent bending of the stick is due to the refraction of light when it passes from water into air. Let us understand it more clearly with the help of a ray-diagram.
Figure shows a straight stick AO whose lower portion BO is immersed in water. Though the stick is actually straight but on immersing in water, it appears to be bent at point B, in the direction BI (see Figure). This can be explained as follows : A ray of light OC coming from the lower end O of the stick passes from water into air at point C and gets refracted away from normal in the direction CX (because it passes from a denser medium water into a rarer medium air). Another ray of light OD gets refracted in the direction DY.
The two refracted rays CX and DY, when produced backwards, appear to meet at point I, nearer to the water surface than point O (see Figure). Thus, I is the virtual image of the end O of the stick which is formed by the refraction of light on going from water to air. Thus, an eye at position E sees the end O of the stick at position I which is nearer to the water surface.
We can extend this reasoning to all the points which make up part BO of the stick under water. Thus, due to the refraction of light, a virtual image of the part BO of the stick is formed at BI. So, what we see under water is actually the virtual image BI of the part BO of the stick under water. Since the part AB of the stick which is above water and the image BI under water are not in the same straight line, the stick AO appears to be bent at point B along BI. Thus, though the actual stick is ABO, it appears to be ABI. Please note that it is not the stick that is really bent. It is actually the light passing from water to air that is bent (or refracted) (see Figure).
When a pencil is partly immersed in water and held obliquely to the surface, the pencil appears to bend at the water surface (when viewed from the side) (see Figure). This apparent bending of the pencil is due to the refraction of light when it passes from water into air. We can use Figure for explaining the bending of pencil in water. Just make a pencil in place of stick.
2. An Object Placed Under Water Appears to be Raised
We will perform an experiment to show that an object placed under water appears to be raised. This can be done as follows : We place a coin at the bottom of an empty basin (a shallow vessel). Let us move our head away from the basin slowly, till the coin disappears from our view and we cannot see it [see below Figure]. Now, without moving our head, we pour water into the basin. We will find that on adding water, the coin appears to rise and we are able to see it [see below Figure]. The coin under water appears to be raised (and becomes visible) due to the refraction of light which takes place when it goes from water into air. We will now understand all this more clearly with the help of a ray-diagram.
Figure shows a coin O placed in an empty basin (having no water). In this case the rays of light OA and OB coming from the coin travel in straight line paths in air and do not enter our eye E (because the eye is at a lower level). Since the rays of light coming from the coin do not enter our eye, we cannot see the coin from this position of our eye.
When the coin is under water, then the rays OA and OB coming from the coin travel in water in straight line paths until they reach the surface of water. When the rays of light OA and OB travelling in water, go out into air, they get refracted (they change their directions). The ray of light OA gets refracted at point A, bends away from the normal, and goes in the direction AX [see Figure]. Similarly, the ray of light OB gets refracted at point B, bends away from the normal, and goes in the direction BY.
If we extend the refracted rays AX and BY backwards (to the left side), then a virtual image of the coin is formed at point I, nearer to the surface of water [see Figure], The refracted rays AX and BY, which appear to be coming from the virtual image of the coin can enter our eye at position E due to which the coin becomes visible to us. Thus, when the coin is under water then due to refraction of light, a virtual image of the coin is formed nearer to the water surface. And since the virtual image of coin which we see, is nearer to the water surface, so the coin appears to rise on adding water in the basin. Thus, it is due to the refraction of light that a coin placed at the bottom of a container appears to rise as the container is slowly filled with water.
If instead of water, we take some other transparent liquid such as kerosene or turpentine in the above two experiments, then the bending of stick or raising of coin will appear to take place by different amounts than that in water. This is because light is refracted by different amounts in different liquids.
3. A Pool of Water Appears to be Less Deep than it Actually is
If we look into a pool of water (or tank of water), it appears to be less deep than it really is. This is due to the refraction of light which takes place when light rays pass from the pool of water into the air. Let us understand it more clearly with the help of a ray-diagram.
Figure shows a pool of water (or a tank of water). Let us take any point O at the bottom of this pool. This point is under the surface of water. Our eye sees this point by the light rays coming from it. Now, a ray of light OA coming from the point O is travelling in water and it comes out into the air at point A. It gets refracted away from the normal in the direction AX (see Figure) Similarly, another ray of light OB, coming from the point O gets refracted at point B and goes away from the normal in the direction BY. The two refracted rays AX and BY, when produced backwards, meet at point I under water. In fact, when the rays AX and BY enter the eye E, they appear to be coming from point I.
So, point I is the virtual image of point O (which is at the bottom of the pool). It is clear from Figure that the image I is nearer to the surface of water than the point O. Thus, a point O at the bottom of a pool appears to be much nearer at position I.
This reasoning can be applied to all the points which make up the bottom PQ of the pool, so that due to refraction of light, the bottom PQ of the pool will appear to be much nearer at the position P’Q’ (see Figure). Due to this, the pool of water will appear less deep than it actually is.
Please note that when we look into a pool of water, we do not see the actual bottom of the pool, we see a virtual image of the bottom of the pool which is formed by the refraction of light coming from the pool water into the air. And since the image of the bottom of the pool is formed nearer to us, we feel that the pool is less deep. From this discussion we conclude that the bottom of a pool of water appears raised due to refraction of light. And the pool of water appears to be less deep than it actually is.
We should be careful while entering a swimming pool because the water in it will be deeper than it appears to be.
It is also due to the refraction of light that a thick glass slab appears to be less thick (when seen from above), than it actually is. Similarly, an ink mark or the writing on a piece of paper appears to be raised and much nearer than it actually is, when viewed by keeping a glass slab over it (see Figure). This also happens due to refraction of light. As we will study in the next Chapter, the stars appear to twinkle on a clear night due to the refraction of light in the atmosphere. Before we go further and discuss the laws of refraction of light.